llvm/lib/Transforms/IPO/SyntheticCountsPropagation.cpp

   1 //=- SyntheticCountsPropagation.cpp - Propagate function counts --*- C++ -*-=//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements a transformation that synthesizes entry counts for
  10 // functions and attaches !prof metadata to functions with the synthesized
  11 // counts. The presence of !prof metadata with counter name set to
  12 // 'synthesized_function_entry_count' indicate that the value of the counter is
  13 // an estimation of the likely execution count of the function. This transform
  14 // is applied only in non PGO mode as functions get 'real' profile-based
  15 // function entry counts in the PGO mode.
  16 //
  17 // The transformation works by first assigning some initial values to the entry
  18 // counts of all functions and then doing a top-down traversal of the
  19 // callgraph-scc to propagate the counts. For each function the set of callsites
  20 // and their relative block frequency is gathered. The relative block frequency
  21 // multiplied by the entry count of the caller and added to the callee's entry
  22 // count. For non-trivial SCCs, the new counts are computed from the previous
  23 // counts and updated in one shot.
  24 //
  25 //===----------------------------------------------------------------------===//
  26
  27 #include "llvm/Transforms/IPO/SyntheticCountsPropagation.h"
  28 #include "llvm/Analysis/BlockFrequencyInfo.h"
  29 #include "llvm/Analysis/CallGraph.h"
  30 #include "llvm/Analysis/SyntheticCountsUtils.h"
  31 #include "llvm/IR/Function.h"
  32 #include "llvm/IR/Instructions.h"
  33 #include "llvm/IR/Module.h"
  34 #include "llvm/Support/CommandLine.h"
  35
  36 using namespace llvm;
  37 using Scaled64 = ScaledNumber<uint64_t>;
  38 using ProfileCount = Function::ProfileCount;
  39
  40 #define DEBUG_TYPE "synthetic-counts-propagation"
  41
  42 namespace llvm {
  43 cl::opt<int>
  44     InitialSyntheticCount("initial-synthetic-count", cl::Hidden, cl::init(10),
  45                           cl::desc("Initial value of synthetic entry count"));
  46 } // namespace llvm
  47
  48 /// Initial synthetic count assigned to inline functions.
  49 static cl::opt<int> InlineSyntheticCount(
  50     "inline-synthetic-count", cl::Hidden, cl::init(15),
  51     cl::desc("Initial synthetic entry count for inline functions."));
  52
  53 /// Initial synthetic count assigned to cold functions.
  54 static cl::opt<int> ColdSyntheticCount(
  55     "cold-synthetic-count", cl::Hidden, cl::init(5),
  56     cl::desc("Initial synthetic entry count for cold functions."));
  57
  58 // Assign initial synthetic entry counts to functions.
  59 static void
  60 initializeCounts(Module &M, function_ref<void(Function *, uint64_t)> SetCount) {
  61   auto MayHaveIndirectCalls = [](Function &F) {
  62     for (auto *U : F.users()) {
  63       if (!isa<CallInst>(U) && !isa<InvokeInst>(U))
  64         return true;
  65     }
  66     return false;
  67   };
  68
  69   for (Function &F : M) {
  70     uint64_t InitialCount = InitialSyntheticCount;
  71     if (F.isDeclaration())
  72       continue;
  73     if (F.hasFnAttribute(Attribute::AlwaysInline) ||
  74         F.hasFnAttribute(Attribute::InlineHint)) {
  75       // Use a higher value for inline functions to account for the fact that
  76       // these are usually beneficial to inline.
  77       InitialCount = InlineSyntheticCount;
  78     } else if (F.hasLocalLinkage() && !MayHaveIndirectCalls(F)) {
  79       // Local functions without inline hints get counts only through
  80       // propagation.
  81       InitialCount = 0;
  82     } else if (F.hasFnAttribute(Attribute::Cold) ||
  83                F.hasFnAttribute(Attribute::NoInline)) {
  84       // Use a lower value for noinline and cold functions.
  85       InitialCount = ColdSyntheticCount;
  86     }
  87     SetCount(&F, InitialCount);
  88   }
  89 }
  90
  91 PreservedAnalyses SyntheticCountsPropagation::run(Module &M,
  92                                                   ModuleAnalysisManager &MAM) {
  93   FunctionAnalysisManager &FAM =
  94       MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
  95   DenseMap<Function *, Scaled64> Counts;
  96   // Set initial entry counts.
  97   initializeCounts(
  98       M, [&](Function *F, uint64_t Count) { Counts[F] = Scaled64(Count, 0); });
  99
 100   // Edge includes information about the source. Hence ignore the first
 101   // parameter.
 102   auto GetCallSiteProfCount = [&](const CallGraphNode *,
 103                                   const CallGraphNode::CallRecord &Edge) {
 104     Optional<Scaled64> Res;
 105     if (!Edge.first)
 106       return Res;
 107     CallBase &CB = *cast<CallBase>(*Edge.first);
 108     Function *Caller = CB.getCaller();
 109     auto &BFI = FAM.getResult<BlockFrequencyAnalysis>(*Caller);
 110
 111     // Now compute the callsite count from relative frequency and
 112     // entry count:
 113     BasicBlock *CSBB = CB.getParent();
 114     Scaled64 EntryFreq(BFI.getEntryFreq(), 0);
 115     Scaled64 BBCount(BFI.getBlockFreq(CSBB).getFrequency(), 0);
 116     BBCount /= EntryFreq;
 117     BBCount *= Counts[Caller];
 118     return Optional<Scaled64>(BBCount);
 119   };
 120
 121   CallGraph CG(M);
 122   // Propgate the entry counts on the callgraph.
 123   SyntheticCountsUtils<const CallGraph *>::propagate(
 124       &CG, GetCallSiteProfCount, [&](const CallGraphNode *N, Scaled64 New) {
 125         auto F = N->getFunction();
 126         if (!F || F->isDeclaration())
 127           return;
 128
 129         Counts[F] += New;
 130       });
 131
 132   // Set the counts as metadata.
 133   for (auto Entry : Counts) {
 134     Entry.first->setEntryCount(ProfileCount(
 135         Entry.second.template toInt<uint64_t>(), Function::PCT_Synthetic));
 136   }
 137
 138   return PreservedAnalyses::all();
 139 }